1. Field of the Invention
The invention relates to a control of particulate matters produced from an internal combustion engine and, more particularly, from a gasoline engine.
2. Description of the Related Art
Particulate matter in engine exhaust gas, including small particles of soot, and the like, is typically a problem in diesel engines. Various technologies for removing particulate matter from diesel engine exhaust gas have been developed. Such a technology is disclosed in, for example, Japanese Examined Patent Application Publication No. 7-106290.
However, particulate matter is produced not only from diesel engines, but also from gasoline engines. In particular, direct-injection type gasoline engines are apt to produce smoke due to excessively high fuel concentration near ignition plugs during a stratified lean combustion mode in which a small amount of fuel provided in a stratified state in combustion chambers is burned. Therefore, there is a strong demand for appropriate removal of particulate matter present in the smoke. Since gasoline engines differ from diesel engines in fuel, and also in engine operating conditions due to the fuel difference, it is necessary to consider removal of particulate matter specifically for gasoline engines.